The goal of this research is to advance the understanding of the limitations, diversity, and comparative aspects of hearing in birds. The study of avian auditory sensitivity has provided a number of insights into the more general mechanisms of hearing in vertebrates. Bird hearing is similar to that of humans in many respects and has served as a useful model for some aspects of human hearing. However, there are important differences. It has long been known that most birds have limited high frequency hearing. Given the comparisons made between bird and human hearing, it is important to know the full extent of the limitations of avian auditory sensitivities in order for accurate parallels and contrasts to be made. This work is intended to fill several gaps in our understanding of the auditory sensitivity of birds. The approach taken blends two complementary avenues of inquiry, the integration of (1) behavioral and (2) physiological methods for evaluating the auditory sensitivity of birds that use high-frequency acoustic communication signals. The specific aims of this application are to examine the relationship between hearing ability and species-specific communication signals in birds, and secondarily to explore the effects of phylogenetic lineage and body size on sensitivity of the auditory system in birds. Results from the proposed experiments will enhance our knowledge of the relationship between the vocal production of learned acoustic communication signals and auditory perception. In the first series of experiments, operant discrimination tests and physiological tests employing auditory evoked potentials will be used to measure basic hearing abilities in sparrows that produce exclusively high frequency (> 6 kHz) sounds. These vocalizations are in frequency regions that most other small birds, including related species, do not hear well. Results will provide information regarding the plasticity of hearing abilities in a group of birds with extremely diverse vocalizations and will direct future research on the potential limitations imposed on a bird's behavior by the match between vocal structure and hearing ability. In a parallel series of experiments, the auditory brainstem response (ABR) will be used to measure hearing abilities in hummingbirds as a prelude to developing psychophysical methods. Hummingbirds are the world's smallest birds and their hearing is completely unstudied. Furthermore, they exhibit vocal learning as in songbirds, but they are phylogenetically unrelated to songbirds. Results from this second set of experiments would thus provide a valuable addition to our general comparative knowledge about auditory abilities in birds, the relationship of size to hearing ability, and the relationship of vocal production to auditory perception.